As a method for manufacturing single-wall carbon nanotubes with high quality, a chemical vapor deposition (CVD) method is expected to be a promising technique. This is because the control of catalyst exhibits the possibility of being able to control the growth of the single-wall carbon nanotubes.
Typically, manufacturing the single-wall carbon nanotubes requires the catalyst metal, such as iron, cobalt and nickel in addition to a carbon material. In the method for manufacturing the single-wall carbon nanotubes based on a conventional vapor phase growth, the nano-particle (fine particle) of metallic oxide such as alumina or silica, or the material having porosity structure such as zeolite is used as the support of the catalyst. This support together with the salt such as the salt of iron is dissolved into solvent, and the catalyst solution is adjusted. After that, this catalyst solution is coated and dried on a substrate, and used as the catalyst for formation of the single-wall carbon nanotubes.
Nature, Vol. 395, Page 878, (1998) discloses the example of using iron nitrate 9-hydrate (Fe(NO3)3.9H2O) and molybdenum acetylacetonate (Mo(acac)2) as the catalyst salts and using the catalyst solution of using alumina nano particle as the support. Then, the single-wall carbon nanotubes are synthesized with methane gas as a carbon supply source. Also, as another example, Chemical Physics Letter, Vol. 296, Page 195, (1998) discloses a method for CVD-growing the single-wall carbon nanotubes by using the catalyst solution, in which the salt of iron, the salt of Mo and alumina nano-particle are respectively used as the catalyst and the support, with CO as a carbon supply source.
However, in the method for using the catalyst solution as mentioned above, it is difficult to selectively grow the single-wall carbon nanotubes. In order to form the catalyst having a predetermined pattern, resist having a predetermined opening is used as mask. As such a method, for example, there are the following two methods.
The first method is a so-called lift-off method. At first, the resist is formed over silicon substrate surface. After that, the resist on the place where the carbon nanotubes will be formed is removed. Next, the catalyst solution is coated over the substrate surface and dried. After that, the substrate is dipped into the solvent of the resist so that the catalyst deposited on the resist is removed together with the resist. This method is typically used in patterning the metallic thin film.
In the second method, firstly, the catalyst solution is coated over the substrate and dried. After that, the resist is coated on the substrate. In succession, the patterning is performed on the resist. In this case, the resist on the portion where the carbon nanotubes will be grown is left. After that, a proper method is used to remove the portion of the catalyst which is not covered with the resist.
However, the case of the first method needs to use the solution which can not solve the resist, as the solvent of the catalyst solution. Also, it was very difficult to deposit the catalyst with a desirable thickness on the portion, in which the resist of the resist pattern does not exist. Actually, the example of dissolving the nano-particles of the alumina, the iron nitrate 9-hydrate (Fe(NO3)3.9H2O) and the molybdenum acetylacetonate (Mo(acac)2) in methanol, and patterning a poly-methyl methacrylate (PMMA) resist is described in the above-mentioned Nature, Vol. 395, Page 878, (1998). However, in this method, the edge portion of the patterned catalyst is unclear. In addition, the pattern of the resist is not also reflected in the pattern of the catalyst. From this fact, it is understood that the method of using the nano-particle has the problem in the control property of the pattern formation.
Also, in the case of using the second method, the dry etching or the wet etching may be considered as the method of removing the catalyst and support in the portion where the carbon nanotubes are not formed. However, the etching resistance of the resist is lower than the iron and the alumina. Thus, currently, it is difficult to etch the iron and the alumina while keeping the pattern of the resist.
Moreover, in the above-mentioned first and second methods, in addition to the above-mentioned problems, the adhesive property between the catalyst support film and the substrate is not sufficiently obtained. Hence, they have the problem that the catalyst support film is easily stripped.
As the method for solving the problems in the method of using such solutions, a method for growing the carbon nanotubes is known, in which the carbon nanotubes are grown from the catalyst film formed on the substrate by using the dry process such as the depositing method. The conventional technique for forming the catalyst thin film by using the dry process will be described below.
The Japanese Laid Open Patent Application JP 2001-20072A relates to a method for forming a large quantity of high purity carbon nanotubes, which are vertically arrayed on a substrate, at a low temperature equal to or less than a deformation temperature of a large area substrate. This gazette describes the process for forming a catalyst metal film on the substrate, then forming the catalyst metal particles of nano-size which are separated by etching the catalyst metal film, and continuously forming a plurality of carbon nanotubes arrayed on the substrate by using a thermal chemical vapor deposition method. The insulating film, such as silicon oxide film, alumina film or the like, may be formed, in order to protect silicide film from being formed by the mutual reaction between the catalyst metal film and the substrate, on the lower portion of the catalyst metal film.
Also, Japanese Laid Open Patent Application JP 2002-115070A and Japanese Laid Open Patent Application JP 2002-115071A describe a technique for forming the thin film of non-catalyst metal, such as Ni and Cu, as a foundation layer, then forming the catalyst metal thin film, such as Fe and Co, in a predetermined pattern thereon, and continuously growing a graphite nano-fiber on this thin film pattern. According to this gazette, alloying is generated between the catalyst metal thin film and the non-catalyst metal, and the adhesive property between both of the thin films is improved, and the adhesive property between the foundation layer and the substrate is also improved. Also, Japanese Laid Open Patent Application JP 2002-115071A discloses a structure in which a thin film pattern of Fe is formed on a Ni thin film, and the carbon nanotubes are formed only on this Fe pattern.
However, in the techniques described in the above-mentioned gazettes, it was not always easy to obtain the sufficiently high yield. Recently, the fact that when iron thin film is deposited on a sapphire single crystal substrate, the single-wall carbon nanotubes can be grown is announced by the inventor of the present invention (contributed to Chemical Physics Letters (2002)). This fact indicates that the growth of the single-wall carbon nanotubes is influenced by the substance supporting the catalyst, namely, the synergistic effect between the support and the catalyst metal has the important meaning. However, in the techniques described in the above-mentioned gazettes, the foundation layer for carrying the catalyst is not designed from the viewpoint of the yield of the carbon nanotubes as mentioned above. The foundation layer is formed from the viewpoint of the protection of the silicide reaction (Japanese Laid Open Patent Application JP 2001-20072A) or the improvement of the adhesive property between the catalyst metal and the substrate (Japanese Laid Open Patent Application JP 2002-115070A and Japanese Laid Open Patent Application JP 2002-115071A), respectively. Thus, the conventional techniques still have the room for the improvement, with regard to the point that the carbon nanotubes are grown at the high yield.
Also, all of the techniques described in the above-mentioned gazettes are intended to be applied to the field of a field emission display and the like. For this reason, so as to be easily applied to a device, multi-wall carbon nanotubes are grown in a direction vertical to the substrate. Thus, they did not give the useful knowledge to the technique for applying to the device, in which the carbon nanotubes are grown in a direction horizontal to the substrate, especially, to the technique for using the single-wall carbon nanotubes.
On the other hand, recently, the development of the transistor using the single-wall carbon nanotubes has been vigorously performed. The transistor of a top gate type using the single-wall carbon nanotubes is described in APPLIED PHYSICS LETTERS, VOLUME 80, NUMBER 20, 20 MAY 2002.
This transistor has a source electrode, a drain electrode and a single-wall carbon nanotubes placed between them, and has the configuration that a gate electrode is formed on these carbon nanotubes.
This transistor is manufactured through the following steps. At first, the silicon oxide film is formed on the silicon substrate, and the carbon nanotubes are dispersed thereon. For example, the carbon nanotubes dispersed in the solution are spin-coated on the silicon oxide film. Next, a probe of AFM (Atomic Force Microscope) is used to determine the position of the carbon nanotubes. After that, electronic beam exposure is carried out to generate the source electrode and the drain electrode. Titanium is used for the configured insulation film of the source electrode and drain electrode. After the source electrode and drain electrode formation, annealing is carried out to form TiC. Consequently, the adhesive property between the source electrode and drain electrode and the carbon nanotubes is improved. After that, the insulating film is formed on the source electrode, the drain electrode and the carbon nanotubes, and the gate electrode is further formed thereon. As mentioned above, the transistor using the carbon nanotubes is completed.
However, in this transistor, it was difficult to maintain the contact resistance between the source and drain electrodes and the carbon nanotubes at a stable low resistance. In the above-mentioned conventional technique, this point is improved by using the titanium as the electrode material and trying the alloying with the carbon nanotubes. However, in order to attain the fast operable transistor, the much lower contact resistance is desired. Also, the conventional technique, since requiring the complex steps to position the carbon nanotubes, still have the room for the improvement with regard to the point of the yield.
In conjunction with the above description, Japanese Laid Open Patent Application JP 2002-105765A discloses the technique of a method for manufacturing carbon nano-fiber composite and carbon nano-fiber. This technique is intended to grow a high dense carbon nano-fiber on a substrate surface.
The method for manufacturing the carbon nano-fiber in this technique firstly reduces the substrate having the interphase of transition metal and interphase of hard reducing metal oxide in reduced atmosphere to extract the transition metal in the interphase, next brings carbon inclusion gas into contact with the extracted transition metal, and then grows the carbon nano-fiber from the extracted transition metal. The substrate having the interphase may be formed by heating the mixed powder containing the oxide powder having the transition metal and the hard reduced metal oxide power up to a reaction temperature.
Japanese Laid Open Patent Application JP 2002-180252A discloses the technique of the method of manufacturing the carbon nanotubes. This method is intended to provide the method for manufacturing the carbon nanotubes grown in a uniform direction on a substrate, in the method for the carbon nanotubes based on the CVD method.
In the method for manufacturing the carbon nanotubes in this technique, the carbon nanotubes are formed on the active substrate by sending the gas of organic carbon raw material onto the active substrate and thermally dissolving it at a temperature between 1100 and 1250° C., wherein the active substrate is formed by depositing the catalyst metal on the substrate at a rate of 0.001 to 0.005 mol/m2. The catalyst metal may be at least one kind selected from Pd, Fe, Co and Ni.
Japanese Laid Open Patent Application JP 2002-146534A discloses the technique of the method for manufacturing the carbon nanotubes. This technique is intended to enable the carbon nanotubes to be arrayed flatly and uniformly at the high dense and also enable the arrayed situation to be easily controlled.
The method for manufacturing the carbon nanotubes in this technique firstly forms a metallic ultra thin film of an iron group metal on an insulting substrate, next charges the insulating substrate and the metallic ultra thin film electrically, then heats and melts the metallic ultra thin film, continuously forms the ultra fine particles separated from each other made of the metal constituting the metallic ultra thin film on the insulating substrate, next decreases the temperatures of the ultra fine particles of the metal and the insulating substrate, then solidifies the ultra fine particles, consequently fixes on the insulating substrate, next discharges electrically the insulating substrate and the ultra fine particle, then supplies the carbon to the insulating substrate and the ultra fine particle by using a vapor growing method, and consequently forms the carbon nanotubes having the metallic ultra fine particle at a tip. The iron group metal may be Ni.
Japanese Laid Open Patent Application JP 2001-176431A discloses a field emission display element and a method for manufacturing the same. This technique is intended to provide the field emission display element that uses the carbon nanotubes which are vertically oriented.
The method for manufacturing the field emission display element in this technique includes: a step of forming a first metal film for a cathode electrode on a lower substrate; a step of growing the vertically oriented carbon nanotubes on the first metal film; a step of placing a first spacer on the first metal film; a step of forming a second metal film as a gate electrode in a shape of mesh heldby the first spacer and formedon the carbon nanotubes; a step of forming a second spacer on the first spacer; and a step of attaching the upper substrate, on which a transparent electrode and a fluorescent material are deposited, on the second spacer. The first metal film may be a chrome film, a tungsten film or an aluminum film, and the second metal film may be a chrome film or a palladium film. The catalyst metal film may be made of cobalt, nickel, iron, yttrium or the alloy of them.
Japanese Laid Open Patent Application JP 2001-48512A discloses a technique of a method for manufacturing a vertically oriented carbon nanotubes. This technique is intended to provide the method that can directly form the carbon nanotubes on the substrate, and control the average values of diameters and lengths of the carbon nanotubes, and further orient the carbon nanotubes vertically to the substrate and selectively manufacture on the substrate or any portion on the substrate.
The method for manufacturing the carbon nanotubes in this technique manufactures the carbon nanotubes oriented in the direction vertical to the substrate surface on the substrate surface by using a plasma CVD method. The substrate may be made of Ni, Fe and Co or the alloy composed of at least two kinds of those metals.